if cfg['TOPO'].getboolean('topoAdd'):
            zv = zv + cExtent*c0*(1. - np.sqrt(np.abs(1. - (np.square(y) / (cExtent**2)))))*mask
            # outside of the channel, add layer of channel thickness
            mask = np.zeros(np.shape(y))
            mask[np.where(abs(y) >= cExtent)] = 1